Storage box for an object to be protected against physicochemical contamination

ABSTRACT

The invention relates to a storage box for an object that is to be protected from physicochemical contamination. The storage box is intended to have low weight, good mechanical strength, a good electrical condition, with a low degassing rate in time with prevention of diffusion of gasses from the external atmosphere into the interior of the box. The inner and/or outer surface of the box&#39;s walls are coated with at least one protective layer. The box may be used to store silicon wafers.

The invention relates to a storage box for an object to be protectedagainst physicochemical contamination.

More specifically, the invention relates to the field of the manufactureof products in an ultra-clean environment. This production uses cleanroom technology consisting of treating the atmosphere in which theproduct is produced. In the agroalimentary or pharmaceutical, as well asthe microelectronics industries, numerous products are consequentlyproduced in a clean room atmosphere in order to avoid contaminationrisks. In the microelectronics field, contamination is more particularlyfeared in the production of parts having very fine geometries and usingthin films such as LCD's or sensors and in the manufacture ofsemiconductor devices, such as microprocessors, static or dynamicmemories, etc.

There are essentially two types of contamination, namely particulatecontamination and physicochemical contamination.

Particulate contamination is due to a physical deposit on the productproduced and which is liable to give rise to physical phenomena. Thus,in the microelectronics field, e.g. on silicon wafers, such deposits canlead to short circuits or to interruptions or disconnections toelectrical connections. In this field, the size of the active geometriesdecreases every year and has passed from a few microns in the 1970's tosubmicron dimensions at the start of the 1990's. Industrialists now havetechnical equipment enabling them to manufacture electronic componentswith geometries of 0.2 to 0.5 μm. A particulate contamination of suchcomponents cannot be avoided by simply using conventional clean roomprocedures.

Physicochemical contamination can be due to the actual productionprocesses, i.e. cleaning or annealing at high temperatures under achemically active atmosphere. It can also be due to contact of theproduced product with its direct environment by mechanical friction onthe product support or by interacting with the surrounding atmosphere,e.g. oxidation during storage between two stages of the process.

In order to avoid contamination, at present two different procedures areused, namely the control of the environment in the manufacturingworkshop or the selective control of the environment of the product.

The control of the environment in the manufacturing workshop consists ofdealing with all the environmental conditions concerning the productionequipment, the products produced and the human operators. This is themost widely used solution. However, due to the presence of the humanoperator, a source of significant chemical and particulatecontamination, this procedure is limited to a cleanness equivalent toclass 0.5 to 1. (Class 1 corresponds to the presence of less than oneparticle, whose diameter exceeds 0.5 μm per cubic foot: Federal standard209c, “Airborne Particulate Cleanliness Classes in Clean Rooms and CleanZones”). The quality of such an environment is difficultly compatiblewith the manufacture of films having geometries lower than 0.20 μm.

Moreover, in such a manufacturing workshop where all the atmosphere iscontrolled, the manufactured product is generally transported in a boxor container ensuring a good protection against particulatecontamination, but a mediocre seal with respect to the surroundingatmosphere. Therefore the gas of the workshop atmosphere, polluted bythe human operator, tends to diffuse through the wall of the containeror box and leads to a high chemical contamination.

The selective control of the environment of the product consists ofsolely optimizing the conditions around the manufactured product. Inthis case, the product is placed within a container, whose internalatmosphere is free from particulate contamination. The advantage of thissolution is that it is theoretically possible to arrive at a systemcomplying with the best possible specifications as regards chemicalcontamination, because the manufactured product is no longer in contactwith the surrounding air and the human operator is then no longer aparticulate and chemical contamination source. The control of theenvironment of the product during the transportation and storage phasestaking place during its manufacturing process is consequently made mucheasier and the chemical contamination is then mainly dependent on theimpermeability performance characteristics of the container.

In the microelectronics field and in the manufacture of silicon wafers,this solution is known as standard mechanical interface or SMIF.

FR 2 697 000 also envisages producing a flat, active confine boxcontrolling the environment of the product during the open and storagephases of said box. More specifically, said box is equipped with anaeraulic unit having a diffuser issuing into the interior of the box andwhich can be connected to an inert gas, e.g. nitrogen supply means.

In the microelectronics field, the storage box for a product such as asilicon wafer must comply with the following specifications:

low unit production cost,

impossibility of using polluting materials, such as e.g. aluminium, ironor stainless steel,

good electrical conduction to obviate problems associated with staticelectricity discharges through the transported product,

low degassing rate in time,

easy cleaning,

good mechanical strength and

low weight permitting human handling.

In order to comply with these requirements, virtually all storage boxesare manufactured from plastics, e.g. polycarbonate or polypropylene.Although these materials comply with the above requirements, it has beenfound that they do not meet the future requirements as regards thepermeability of contaminating agents, whose diffusion through plasticsmaterial is fast, even at ambient temperature.

The object of the invention is to develop a storage box meeting theaforementioned specifications and solving the problems of the prior art.It therefore relates to a storage box for an object to be protectedagainst physicochemical contamination. The walls of said box are madefrom a plastics material.

According to the characterizing part of the invention, the inner surfaceand/or outer surface of the walls of said box is coated with at leastone protective layer of a material having the general formula:

SiO_(x)N_(y)H_(t),

where t is lower than x and/or y and x and y are preferably in thefollowing ranges:

0<x<2

0≦y<0.4

Preferably, x is in the range 0.3 to 1.8.

The contribution of hydrogen to the composition SiO_(x)N_(y)H_(t) willusually be very low, essentially coming from the gaseous precursor ofthe silicon used for performing the deposit, which is in general amolecule containing hydrogen. Therefore, in all cases t is less than atleast one of the parameters x, y and x only in the case where y is zero(no nitrogen contribution to the material).

As a result of the characteristics of the invention, a storage box isobtained in which the chemical contamination resulting from thediffusion of gases through the plastic material wall is verysignificantly reduced.

In the case where deposition takes place on the inner wall of the box,the organic chemical contamination due to the degassing of the plasticsmaterial walls towards the inside of the box kept under a vacuum isreduced. There is also an improvement to the surface state of theinterior of the box and its cleaning is facilitated. There is also animprovement to the mechanical strength of the inner surface of the thustreated box and a reduction of the particulate contaminationmechanically produced by impacts or scratches.

Preferably, the protective layer is deposited on said surface orsurfaces of the box by plasma enhanced chemical vapour deposition(PECVD). This procedure makes it possible to make homogeneous depositsof limited thickness on a plastics material. The thickness of theprotective layer is advantageously at least 0.1 μm. The minimumthickness of the layer is imposed on the one hand by specificationslinked with permeation and on the other by the need for a mechanicalstrength.

According to an advantageous embodiment of the invention, the thicknessof the layer is equal to or greater than 1 μm and is preferably in therange 2 to 3 μm.

Plasma enhanced chemical vapour deposition methods are described in FR 2631 346, EP 502 790 and EP 519 784.

The reactor and processes described in these patents were developed inorder to obtain high density layers (little or no columnar or granularstructures) operating at quasi-ambient temperature. Thus, if a plasticsmaterial such as polycarbonate only undergoes irreversible modificationsas from 115 to 120° C., the inorganic layer will generally be crackedand/or delaminated well before said limit due to the very high thermalstresses applied to the interface as a result of the significantasymmetry of the expansion coefficients (ratio 10 to 20). Thereforeworking at substrate temperatures above 60° C. is avoided.

Use is generally made of high density plasmas and in most cases it isnecessary to cool the surface of the storage box element on whichdeposition takes place, by keeping the box holder (in contact with theface opposite to the deposit) at a temperature of approximately 10 to20° C.

Another important aspect of such processes is the control of the ionbombardment of the substrate. As working takes place at lowtemperatures, it is necessary to supply non-thermal energy in the formof a kinetic impact of the ions in order to assist the migration andrearrangement of atoms condensing on the surface. Only under thiscondition is it possible to obtain dense microstructure layers offeringgood functional properties (hardness, physicochemical inertia,impermeability, etc.).

Finally, the adhesion of the inorganic layers to the storage box must beexcellent for all applications of thus protected polymers. In thepresent case, the box must be periodically cleanable in a liquidultrasonic bath throughout its life. This is a very severe criterion.The process described in FR 2,631,346 makes it possible to producecoatings, which have a remarkable resistance to ultra-sonic cleaning.This process consists of a succession of an argon plasma, a 10% ammoniaplasma in argon and then a highly helium-diluted silane plasma.

Research carried out by the applicants have revealed that the storageboxes with coatings produced by the processes described in FR 2,631,346,EP 502 790 and EP 519 784 have traces of organic contamination liable todeteriorate the surface of the object (e.g. a silicon wafer) stored insuch a box. However, this contamination is localized in the surfacelayer of the protective coating. It is essentially due to theinteraction of high energy species of the plasma with the polymersubstrate, whose characteristic organic functions in the carbon surfacelayer can be identified.

As a function of the stage considered in the preparation of the objectto be stored (e.g. during multiple stages intervening in the manufactureof a semiconductor device), said organic contamination can definitelyhave a very prejudicial effect. For a better understanding of suchphenomena reference can e.g. be made to the publication of W.Vandervorst et al. in “Proceedings of the 2nd International Symposium onUltra Clean Processing of Silicon Surfaces, Leuven, Belgium, September1995”.

The applicants considered it significant to have a method making itpossible to bring about the removal of said surface carboncontamination. Supplementary research carried out for providing atechnical solution to this problem has demonstrated that it is possibleto achieve such a result by performing a plasma etching stage,preferably immediately following the above-described SiO_(x)N_(y)H_(t)coating deposition phase and in the same reactor.

The invention also relates to a process for the treatment of a storagebox of an object which is to be protected against physicochemicalcontamination, whose walls are made from a plastic material andaccording to which the deposition takes place on the inner surfaceand/or the outer surface of its walls of at least one protective layermade from a material of formula SiO_(x)N_(y)H_(t), in which t is lowerthan x and/or y, by a PECVD procedure, x and y preferably beingrespectively in the ranges 0<x<2 and 0≦y<0.4. More preferably, x is inthe range 0.3 to 1.8.

According to one of the variants of the invention, this is followed by asurface etching of the previously deposited protective layer, bycontacting the box with a plasma of a gaseous mixture based on oxygenand a fluorine gas, the latter being advantageously chosen from amongSF₆ and NF₃.

As there is no selectivity constraint, conditions are adopted giving thehighest etching speed. Information can be obtained in this connectionfrom the literature concerning SF₆ plasma etching of SiO₂. In this case,the etching is not spontaneous and requires a bombardment of the surfaceby accelerated ions from the plasma. This effect is obtained in the sameway as in the case of the process for the deposition of the layers, byapplying to the metal substrate holder on which rests the rear face ofthe element of the polycarbonate storage box and on which the etchingoperation takes place, a radio frequency biasing of typically 13.56 MHz.As a result of self-biasing, there is a continuous negative potentialdifference between the plasma and the surface of the sample.

Specifically, the results available in the literature with respect toSiO₂ etching by SF₆, show that a maximum etching speed is obtained forpartial pressures of SF₆ of 1 to 10 millitorr (0.13 to 1.3 Pa), with anion energy of approximately 100 eV. It is not therefore appropriate towork under conditions where said energy is significantly higher.

The partial oxygen pressure in the etching mixture can be chosen withina relatively wide range without moving far away from the optimumconditions. The problem of the oxidation of the polycarbonate (browningrisk) does not arise here, because the box is already covered withsilica.

The higher the oxygen concentration in the mixture, the higher theetching speed. An oxygen concentration in the range 10 to 80% issuitable in most cases.

Experience has shown that the SF₆/O₂ plasma has an excellent efficiencyfor etching the surface layer, which is a mixture of SiO₂ or SiO_(x) andorganic fractions.

It is preferable to provide a means for controlling the etched depth inorder to only remove the carbon surface layer thickness. To this end, itis possible to adjust the etching speed and stop the treatment after afixed time (for safety reasons significantly increased compared with thenominal value, which leads to an over-etching). It is also possible tomake the process dependent on an end of etching detection means, e.g. anin situ optical measurement.

Tests were performed in the surface wave microwave reactor described inFR 2 677 841. In this type of reactor, whose special feature is that thesubstrate is in the very near post-discharge, it is not indicated todirectly inject SF₆ into the surface wave tube. In a halogen medium, theenergy microwave field of the surface wave gives rise to an extremelyfast etching phenomenon with respect to the silica tube wall (muchfaster than for a silica sample placed in the homogeneous plasma in thecentre of the tube). This etching rapidly leads to a devitrification,which can deteriorate the mechanical strength in vacuo of the reactor.

As a consequence, the surface wave discharge is only maintained in amixture of oxygen and argon. The latter facilitates the control of theextension of the plasma throughout the tube volume. SF₆ is injected inpost-discharge in the vicinity of the sample. With sufficiently highmicrowave powers, it is possible to obtain a very high plasma densitypermitting an effective dissociation of the SF₆ and adequate activehalogen species concentrations (particularly atomic fluorine) in thevicinity of the substrate.

The process parameters are then typically as follows:

microwave power: 400 watts continuous self-biasing of the substrateholder with respect to earth: −100 V

total pressure: 70 millitorr (9.1 Pa)

Ar flow rate: 125 cm³ standard/minute

Ar flow rate: 50 cm³ standard/minute

O₂ flow rate: 50 cm³ standard/minute

SF₆ flow rate: 50 cm³ standard/minute.

The etching speed obtained is typically approximately 100nanometres/minute. The carbon surface layer removal operationconsequently takes less than one minute on average.

As will be clear to the expert, the choice of the reactor used forperforming the deposition of the SiO_(x)N_(y)H_(t) layer or layers orfor carrying out the etching of the organic contamination is important.

Apart from the reactor examples already given, it is pointed out that itis possible to carry out the deposition or etching using a radiofrequency reactor of the planar diode type, like that described inJP-05/202211, or in a microwave reactor based on an applicator of theleak guide type, like that described in U.S. Pat. No. 4,893,584, or alsoin certain plasma reactors especially adapted to the treatment ofsubstrates with a size significantly exceeding 10 cm² and possiblyhaving an awkward shape, such as reactors based on the microwave plasmaexcitation concept using uniformly distributed electron cyclotronresonance (UDECR), like that described in EP-A-496 681.

Reference can also be made to the research relating to PECVD productionof amorphous silica layers of very great uniformity in such a UDECR-typereactor, reported in the article by J. C. Rostaing et al., published inProc. Int. Conf. on Metallurgical Coatings and Thin Films, San Diego,Calif., U.S.A., Apr. 24-28, 1995.

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and the attached drawings, wherein show:

FIG. 1 A storage box according to the invention.

FIG. 2 A graph illustrating the variation of the oxygen concentrationoutside and inside a box according to the invention.

An embodiment of the invention will now be described. As illustrated inFIG. 1, the storage box according to the invention comprises a body 1, adoor 3 and optionally an aeraulic unit 5 making it possible to diffusean inert gas into the interior of the volume of the box and anidentification code 7 giving information regarding the contents of saidbox. The body 1 is advantageously formed by two U-shaped half-shells 9,11 made from a plastics material, e.g. polycarbonate or polypropylenewith a thickness of 2 mm.

The protective layer 13, 14 is respectively deposited on the inner andouter walls of the box 1 prior to the assembly of the two half-shells 9,11. These two half-shells are e.g. assembled with an epoxy resinadhesive or are welded, with the exception of the aeraulic unit 5, whichis fitted onto the remainder of the box 1. The deposition of theprotective layer 13 also takes place on blocking elements and carryingthe object 15 within the box, e.g. wedges or rods (the latter not beingshown in FIG. 1), but such elements are in any case optional.

For illustration purposes, a storage box intended to contain a siliconwafer of diameter 200 mm and thickness 725 mm has the followingapproximate dimensions:

External dimensions:

length: 240 mm

width: 240 mm

thickness: 19 mm

Internal dimensions:

length: 230 mm

width: 210 mm

thickness: 9 mm

The deposition of the protective layer 13 takes place on the inner wallsof the half-shells 9, 11 forming the box, using a UDECR reactor such asis described in the aforementioned publication of J. C. Rostaing et al.This reactor makes it possible to deposit a fine film of silicon oxideand/or nitride and/or oxynitride from a microwave plasma excited at thefrequency 2.45 GHz. This reactor is supplied with the followingprecursor gases:

for silicon: silane (SiH₄), Si₂H₆ or Si₃H₈ or halosilanesSiX_(n)H_(4−n), (with X representing chlorine or fluorine and n≦4),

for oxygen: O₂,

for nitrogen: N₂, NH₃ or N₂O.

The pressure within the reactor is below 10 mTorr (1.33 Pa). Thesubstrate surface temperature does not exceed 100° C., so as not tobring about a deterioration of the surface qualities thereof. Theadhesion qualities of the protective layer 13 to the surface of theplastics material box 1 can be improved by a prior exposure of saidsurface to an argon plasma, e.g. such a pretreatment being described inFR-A-2 631 346.

Table 1 is a typical example of the criteria required in order to obtainthe necessary box coating performance characteristics.

TABLE 1 Characteristics Standards Permeability to O₂ <10⁻¹⁵ cm²/sSurface resistance <10⁹ Ω/² ASTM D-257 Electrostatic discharge <3s MILB-81705B Roughness:Ra <0.2 μm Tensile/shear strength >1 N/mm²Transmittance >75% ASTM D-1003 Inactinic properties cutoff at 0.45 μmKnoop hardness 200 N/mm² (load 0.1 kg) Vickers hardness 120 N/mm²Thickness <10 μm Life 5 years

FIG. 2 illustrates the contamination observed by gaseous diffusionthrough the walls of a standard silicon wafer storage box at ambienttemperature. The abscissa x represents the thickness direction of thewall, h the thickness of the box wall and C the polluting elementconcentration.

According to the first law of Fick (cf. “Procédés de séparation parmembrane”, J. P. Brun, Paris XII University, Masson, 1989), the materialflux F passing through the storage box wall is given by the followingformula: F=−Div C, or per unit section F=−D.dC/dx, D representing thediffusion coefficient.

Assuming that the concentration change remains very low in the box andconsequently does not affect the equilibrium of the concentrations inthe thickness of the wall, it can be considered that the internalconcentration (C_(int)) is well below the external contamination(C_(ext)). It can therefore be estimated per section unit F=−DΔC/Δx.

The concentration variation within the time internal t within a box isgiven by the reentering flux F per section unit S, divided by thecontainer volume, namely: C_(int)=F.S/V.t.

Moreover, on the basis of the starting hypothesis that the consideredcontaminant is the oxygen contained in the external air (C_(ext)≈21%)and that at t=0 the oxygen concentration C_(O) ₂ on the box inner wallis zero, it can be considered that the oxygen concentration in the boxat the end of a time t is given by the following expression:

C_(O) ₂ =C_(ext).S.D.t/V.h

in which S represents the total surface of the inner walls of the box, Drepresents the diffusion coefficient of oxygen in the consideredmaterial at a given temperature, V represents the box volume and hrepresents the thickness of the walls.

Two numerical examples illustrating the advantages of the invention willnow be given.

1) COMPARATIVE EXAMPLE Polycarbonate Box Without Protective Layer

D=21.10⁻⁹ cm².s⁻¹, (at 300 K),

V=320 cm³,

S=800 cm².

h=0.2 cm,

C_(ext)=21%.

With the above values and applying the preceding formula, an oxygenconcentration in the box after one second is

C_(O) ₂ =0.21×800×21×10⁻⁹/320×0.2=55·10⁻⁹ or 55 ppb.

The chemical purity specification in a box compatible with a 1 Gbytememory production technology must be just below 50 ppb. Thus, this valueis reached after only one second, which means that a conventionalpolycarbonate box is not compatible with the specifications required inthe future for microelectronics technologies.

2) EXAMPLE Polycarbonate Box Covered With a 1 μm Thick Silicon OxideProtective Layer

In this case, the diffusion coefficient D of oxygen in silicon oxide isgiven by the extension of the Fick equation, i.e. D=D_(o).e-E/kT, whichE represents the oxygen activation energy in silicon oxide, i.e. in thiscase 1.16 eV, D_(o) represents the oxygen diffusion constant in siliconoxide and is equal to 2.7·10⁻⁴ cm²/s and k represents the Boltzmanconstant (8.62·10⁻⁵ eV/° K), which means at ambient temperature:

D_(300° K)=2.7·10⁻⁴.e-(1.16/8.62·10⁻⁵.300)=10⁻²³ cm⁻²/s.

Using the other numerical values of the comparative example, theconcentration in the box after one second and only considering theoxygen diffusion through the single, 1 μm thick silicon oxide layer (h=1μm) is:

C_(O) ₂ =C_(ext).S.D.t/V.h

C_(O) ₂ =0.21.800.10⁻²³/320.10⁻⁴=5.25·10⁻²⁰ after one second.

Thus, for a polycarbonate container coated with a 1 μm thick silicadeposit, the 50 ppb specification will be obtained after approximately10₁₁ s, i.e. after approximately 3000 years.

However, in practice, the silicon oxide layers deposited by CVD plasmaat ambient temperature contain defects giving rise to faster diffusionconditions. However, there is a considerable margin for maintainingwithin the requisite performance range, i.e. maintaining thespecification of 50 ppb for a few minutes to a few hours, or a few daysor even a few weeks if necessary.

The reasoning and orders of magnitude are identical with boxes coveredwith a layer of silicon nitride or silicon oxynitride, which can bedeposited at low temperatures and which also constitute very gooddiffusion barriers against oxygen. In the same way, these differentprotective layers prevent the diffusion into the box of any other atomiccompound or molecule contained in air, such as e.g. water vapour.

What is claimed is:
 1. A storage box for an object to be protectedagainst physiocemical contamination, wherein the walls comprise aplastics material and the inner surface and/or outer surface of thewalls is/are coated with at least one protective layer consistingessentially of a material formula SiO_(x)N_(y)H_(t), where t<x+y, whenx>0 and y>0; t<x, when y=0; and x>y when y=0, obtained by depositingsaid protective layer by plasma enhanced chemical vapor depositionwherein the source of silicon is a gaseous source of silicon, followedby surface etching the deposited protective layer by contacting the boxwith a plasma of a gaseous mixture comprising oxygen and fluorine,thereby removing surface carbon contamination wherein said box issubstantially impermeable to O₂.
 2. The storage box according to claim1, wherein x is higher than 0 and lower than
 2. 3. The storage boxaccording to claim 1, wherein y is equal to or higher than 0 and lowerthan 0.4.
 4. The storage box according to claim 1, wherein the thicknessof the protective layer is at least 0.1 μm.
 5. The storage box accordingto claim 2, wherein the thickness of the protective layer is at least0.1 μm.
 6. The storage box according to claim 3, wherein the thicknessof the protective layer is at least 0.1 μm.
 7. The storage box accordingto claim 4, wherein the thickness of the protective layer is at least 1μm.
 8. The storage box according to claim 5, wherein the thickness ofthe protective layer is at least 1 μm.
 9. The storage box according toclaim 6, wherein the thickness of the protective layer is at least 1 μm.10. The storage box according to claim 4, wherein the thickness of theprotective layer is between 2 and 3 μm.
 11. The storage box according toclaim 5, wherein the thickness of the protective layer is between 2 and3 μm.
 12. The storage box according to claim 6, wherein the thickness ofthe protective layer is between 2 and 3 μm.
 13. The storage boxaccording to claim 1, wherein the walls comprise polycarbonate orpolypropylene.
 14. The storage box according to claim 1, furthercomprising an aeraulic unit.
 15. The storage box according to claim 1,containing at least one silicon wafer.
 16. A process for the productionof the storage box according to claim 1, comprising depositing theprotective layer on the surface or surfaces of the box walls by plasmaenhanced chemical vapor deposition and surface etching the depositedprotective layer by contacting the box with a plasma of a gaseousmixture comprising oxygen and a fluorine gas selected from the groupconsisting of SF₆ and NF₃.